Over the past thirty-five years, asphalt has replaced concrete as the material of choice for paving roadways. Asphalt provides a smooth road surface that is easier to apply and easier to resurface than concrete. However, asphalt pavers generally cannot pave an entire road surface in a single pass and therefore joints must be formed between the previously placed asphalt and the newly placed asphalt being placed by the paver. It is at these joints that asphalt is most susceptible to failure as the “cold”, previously placed, asphalt is not sufficiently heated by the hot asphalt from the paver to deform during compaction. This leads to confined compaction, in which the new “hot” asphalt is not compacted, but rather is pushed sideways, resulting in a joint having that is not sufficiently dense and homogenously bonded. These non-homogenous “cold joints” often will separate and create cracks, most usually in the center of two lane roads, which allows water to enter the pavement, migrate to the road base and, over a period of time, contribute substantially to base failure of the road.
To overcome this problem, the previously placed pavement joint, must be heated to approximately the same temperature as the new material that is being placed to provide the proper environment for thermal bonding. However, the chemical properties of asphalt make this a difficult task. First, asphalt is a petroleum-based product and has a tendency to burn if overheated. Second, the mass and thermal conductivity of asphalt necessitate heating the asphalt for a significant period of time to raise the temperature of the entire asphalt layer to the desired bonding temperature. The combination of these properties makes traditional heating by propane torch, either hand-held or paver-mounted, an ineffective and dangerous exercise.
When paver-mounted heaters are used to preheat the asphalt, the temperature of the burning propane raises the temperature of the top layer of asphalt to its burning temperature well before the remainder of the asphalt layer has reached its desired bonding temperature. Thus, to avoid igniting the top of the asphalt layer, the paver must move before the entire layer is sufficiently heated, bonding the new hot asphalt to “warm” asphalt. This increases the adhesion somewhat, but does not increase the density of the joint sufficiently to provide the durability obtained by of a true “hot joint”. When hand held heaters are used, the heat to the pavement may be cycled to allow proper heating of the entire layer. However, the risk of fire increases with this approach, as the operator controls the exposure of the heater to the pavement. In addition, the use of hand held heaters requires additional manpower and slows the paving process.
In response to this problem, the inventor of the present invention developed the paving apparatus and method disclosed and claimed in U.S. Pat. No. 6,227,762, which is incorporated herein by reference. This patent describes and claims a method that includes the steps of placing a first layer of asphalt in a roadway and allowing the first layer of asphalt to cool to a temperature below a predetermined minimum bonding temperature. The surface of the joining area of the first layer is then preheated to a temperature above the minimum bonding temperature but below the ignition temperature of the asphalt. The remainder of the joining area of the first layer is then allowed to absorb the heat from the surface. After a predetermined period of time is allowed for absorption of heat, the surface of the joining area of the first layer of asphalt is again heated to a temperature below the ignition temperature of the asphalt and the second layer of asphalt is placed adjacent to the first layer such that a joint is formed between the first layer and the second layer. Extensive testing using this method has confirmed that it is extremely effective at achieving the desired result. This testing has also caused the inventor to improve upon the asphalt heating system for performing the method.
In U.S. Pat. No. 6,227,762, the preferred heating system included one twelve foot infrared heater attached to the side of the paver and a mobile pre-heater “train” composed of several twelve foot infrared joint heating units pulled by a motorized vehicle. It was noted that two to three pre-heater units were sufficient for most paving conditions, as this number of heaters was sufficient to raise the temperature of the surface of the joining area of the first layer, which is then preheated to a temperature above the minimum bonding temperature but below the ignition temperature of the asphalt. However, testing has shown that heating with two to three pre-heater units does not allow the heat from the surface to penetrate downward all the way to the base to produce a full hot joint. Consequently, a hot joint is formed along a layer proximate to the surface and a “warm” joint is formed at a layer adjacent to the base. Such a joint is still a significant improvement over the cold joints of prior methods, as the hot portion of the joint is at the surface, which is where most failures begin. Notwithstanding this fact, the best joint is a full hot joint and, therefore, there remained a need for a system that would produce a full hot joint.
In response to this need, the inventor experimented with a number of pre-heater and post-heater configurations to determine how to heat the asphalt such that heat penetrates fully from surface to base to produce a substantially full hot joint. Testing was conducted utilizing thermocouples embedded at different depths within the asphalt and with continual monitoring of the cycling of the heaters based upon the temperature sensors attached thereto. This testing resulted in the development of the reflector apparatus, kit, asphalt paving system and method of the present invention.